1. Field of the Invention
The present invention relates to a light-emitting semiconductor diode (LED) and a laser diode (LD) that use a Group III nitride compound and that has a multiple emission layer. Especially, the invention relates to an LED and an LD having an improved emission efficiency in the visible short wavelength region from the green blue to blue color and in the ultra violet region.
2. Description of the Related Art
It has been known that an aluminum gallium indium nitride (AlGaInN) compound semiconductor may be used to obtain a light-emitting diode (LED) or a laser diode (LD) which emits blue and ultra violet color light. This semiconductor device is useful because of its high luminous efficiency resulting from direct electron transition and because of its ability to emit blue light, which is one of the three primary colors.
By irradiating an electron beam and carrying out heat treatment, a magnesium (Mg) doped i-layer is changed into a p-type conductive layer. As a result, an LED or an LD is obtained having a double hetero p-n junction structure made of an aluminum gallium nitride (AlGaN) p-layer, a zinc (Zn) doped indium gallium nitride (InGaN) emission layer, and an AlGaN n-layer. Such an LED or LD is more prosperous in the semiconductor market than a conventional LED having a metal insulator semiconductor (MIS) structure which includes an n-layer and a semi-insulating i-layer.
As shown in FIG. 6, an LED 10 is disclosed in Japanese Patent Application No. 113484/1994 (not yet laid open) which has higher luminous emission. A GaInN emission layer 5 of the LED 10 is doped with both zinc (Zn) and silicon (Si), and both planes of the emission layer 5 form a double hetero-junction structure with an adjacent AlGaN n-layer 4 and an AlGaN p-layer 61. The peak wavelength of the LED 10 is between 420 and 450 nm and its luminous intensity is 1000 mcd. Such LEDs with higher luminous intensity of blue color are in great demand, for example, for use in multicolor display devices.
While light, having a peak wavelength of about 500 nm, resulting in a green blue or dark green color, is required for traffic signal lights, the conventional LED 10 is unable to provide that required wavelength. In order to meet that requirement, the energy band width of the emission layer needs to be narrowed by increasing the ratio of indium (In) among the components of the emission layer 5. Furthermore, both an acceptor impurity and a donor impurity are doped into the emission layer 5 controlling their impurity concentrations to maximize luminous intensity of the LED.
Such arrangements, increasing the In ratio among the composites of the emission layer 5 and doping the emission layer with an acceptor and a donor impurity, however, rather raise significant potential energy caused by the Coulomb force between the acceptor and the donor, and the electron transition energy becomes equal to the sum of the potential energy and the energy difference between the acceptor and donor levels. The energy difference between the acceptor and donor levels virtually becomes larger than that in the case of no Coulomb force. As a result, the peak wavelength is shifted toward a shorter wavelength in the luminous spectrum and the required wavelength peak of 500 nm cannot be obtained.
As shown in FIG. 10, a gallium nitride compound semiconductor device 20 with a multiple emission layer structure is disclosed in laid-open Japanese Patent Application number 268257/1994. The emission layer is formed by three In.sub.0.2 Ga.sub.0.8 N 44 wells and two In.sub.0.04 Ga.sub.0.96 N 44' barriers. Each of them has a thickness ranging from 5 to 50 .ANG. and they are laminated alternately.
The peak wavelength of the device 20 of FIG. 10 is still around 410 to 420 nm, because the emission mechanism of the device 20 is an inter-band recombination formed without doping any impurities in the wells 44 which act as a luminous center. Such a peak wavelength does not meet the required 500 nm wavelength for a traffic signal. Further, the luminous intensity of the device 20 still has room for improvement. Therefore, there is a need for an LED having both a larger peak wavelength and higher luminous intensity.
InGaN and AlGaN are representative materials for an emission layer of a Group III nitride compound semiconductor device which emits ultraviolet rays. When InGaN is utilized for the emission layer and the composition ratio of In is 5.5% or less an ultraviolet ray having a peak wavelength of 380 nm is obtained and the emission mechanism of the device is the inter-band recombination. When AlGaN is utilized for the emission layer, the emission layer is doped with Zn and Si, and the composition ratio of Al is around 16%, an ultraviolet ray having a peak wavelength of 380 nm is obtained and the emission mechanism of the device is the electron transition between energy levels of the donor and the acceptor.
Although the peak wavelength of such devices utilizing InGaN or AlGaN is satisfactory, the luminous efficiency of the same is still poor for several reasons. The emission layer made of InGaN has a poor luminous efficiency due to poor crystallinity as a result of low growth temperature and carrier recombination between bands. The emission layer made of AlGaN has a poor luminous efficiency due to a dislocation resulting in a mismatch of lattice constants.